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Billionaire Paul Allen announced he is pledging the bulk of his fortune -- an estimated $13.5 billion -- to charity.

Allen's announcement comes at the heels of fellow Microsoft co-founder Bill Gates and investor Warren Buffett's call for billionaires to pledge the majority of their wealth to philanthropy. He is one of the first to make a public announcement since the challenge.

Allen's relationship with Gates goes back to 1975, when the duo co-founded Microsoft. He had been consulting with Gates in the hopes that the public announcement will help nudge others to give, too, according to The Seattle Times, .

"I've planned for many years now that the majority of my estate will be left to philanthropy to continue the work of the Foundation and to fund nonprofit scientific research," Allen said, according to a Reuters report.

Although Allen, 57, retired from Microsoft in 1983, he's apparently maintained a stake in the company that made him rich and has a long history as an active philanthropist. He is founder of The Allen Institute for Brain Science, a foundation dedicated to research on brain functions and the Experience Music Project a music museum in Seattle and the Paul G. Allen Family Foundation, which celebrated its 20th anniversary today.

By our estimate Allen has already donated $900 million (as of last year) to charitable causes.

If each of the above billionaires give away half their net worth, charities will benefit to the tune of at least $19.4 billion. Yet Gates and Buffett are hoping for $600 billion; it remains to be seen how many other generous billionaires will come forward to meet this challenge.